Adding to the awkwardness of the task is the fact that we have new (not better!) vinyl windows that are not as sturdy as wood windows. For example, we don't really want to drill holes in the vinyl. Nor do we want the weight of a larger a/c resting on mere vinyl.
We've also found that the companies that sell air conditioners do not really address these issues. It's pretty much up to the homeowner to figure it out. And perhaps that's why you see huge units hanging precariously overhead propped up by a few bricks or boards.
After grappling with this issue for the past few years, we have finally found a solution that is less unsightly and also very secure, which is an issue living on the first floor.
This is one of those mundane tasks that we old house people face. It's one of those things I would think of no interest, but then a commenter asked to see our system and yet again, I realize that these are the types of solutions worth sharing.
First: Steve glued up boards to make the proper thickness, 1/4" above the vinyl window frame. This piece will be on the inside window sill supporting the a/c.
Note: Air conditioners should lean outward so that any condensation drips outside and not on your inside sill.
Here are the boards for each window glued up and ready to be cut to size with the table saw.
Next we removed the window stops.
Here you see the stained ledge in place after being cut to size without the window stop.
And with the trimmed windows stops and firmly in place. As Steve said, "This ain't goin' nowhere!"
The larger air conditioner is screwed into the board making it very secure and stable. We will probably add angle irons for further security.
This is a solution we can live with and best of all, it's permanent. All we have to is pop the a/c in or out as the seasons pass. Like us, the support ledge is there to stay.